This invention relates generally to front-end integrated circuit processing methods and equipment, and more particularly to laser interferometer end point detection systems.
Integrated circuits are fabricated in bulk on semiconductor wafers. Typically, scribe lines are provided on the surface of a wafer in a grid pattern, such that the individual integrated circuit or "chips" can be easily separated from each other. After the wafer is fully processed it can be broken or cut along the scribe lines to separate the chips for packaging.
The semiconductor wafer is repetitively subjected to a number of processes during the integrated circuit fabrication procedure such as masking, etching layer formation, and doping. The present invention is primarily concerned with the etching process, i.e. the removal of layers of materials from the surface of semiconductor wafers.
As the density and complexity of integrated circuit increase the sizes of the various features within the integrated circuits necessary decrease. This evolution towards small feature sizes required a high level of control over the etching process. For example, the etching process should be highly anisotropic so as to provide an edge profile with a high aspect ratio. Also, the etching process should be very controllable so that the etching process can be predictably stopped after the layer has been etched through.
Determining when a layer has been etched through can be an open-loop process, i.e. the etching process can be allowed to progress for a predetermined period of time with the hope that the layer will be etched through without too much over-etching. However, since wafers do not always etch at the same rate, there is a tendency to run the etching process long enough to etch through the slowest etching wafers, resulting in over-etching of the faster etching wafers. As feature sizes become smaller, this results in an unacceptably high defect rate and, thus, this open-loop method is not used on state of the art etching equipment.
Most modern etching equipment make some provisions for endpoint detection, i.e. detection of etch-through in a desired layer. One approach which is useful with semitransparent layers such such as silicon dioxide (SiO.sub.2) is to use the principles of laser interferometry. With laser beam interferometry, a laser beam is directed at the layer being etched and a reflected portion of the beam is detected by an appropriate photodetector. Since the etching layer is semi-transparent to the frequency of laser light being used, some of the incident beam will be reflected from the top surface of the layer and some of the beam will be reflected from the bottom surface of layer. These two reflections will either constructively or destructively interfere with each other, creating a characteristic sinusoidal etching curve as the layer is etched away. When the etching curve flattens out, the layer has been etched through and endpoint has been detected.
One such laser interferometer system is described in U.S. Pat. No. 4,618,262 of Maydan et al. which is assigned in common with the present invention to Applied Materials, inc. of Santa Clara, Calif. In the Maydan et al. patent, a process is described which includes scanning a laser beam across scribe lines on a wafer being etched, and monitoring the resultant interference pattern. Alternatively, Maydan et al. teach a process including the scanning of a laser beam across a wafer to find a scribe line, locking the laser beam on to the scribe line, and then monitoring the resulting interference pattern. In both processes, the size of the laser beam spot is comparable to the width of the scribe lines. In other laser interferometer end point detection systems, the laser spot is much larger than the width of the scribe lines.
While laser interferometer systems such as the one taught by Maydan et al. perform admirably, they have been found to be sensitive to various forms of noise. For example it has been found that a great deal of noise is generated at the detector when the laser beam scans over a step or transition on the wafer, and that noise is at a minimum on surfaces having uniform film thicknesses. Such transitions are commonly produced by photomasking processes where a patterned layer of photoresist is developed on top of a layer to be etched. In consequence, when the laser beam spot is comparable to or larger then the width of the scribe lines it is inevitable that a certain amount of noise will be generated as the beam traverses the scribe lines, since a portion of the beam will always be on a transition.
The noise amplitude obtained while traversing a transition can be significantly higher than the amplitude of the etching curves of the flat surfaces. In the past, the laser beam might lock onto such a transition due to this high noise level, resulting in the monitoring of the photoresist or a combination of photoresist and silicon dioxide. The usefulness of a smaller beam size with an adequate depth of field to alleviate this problem was heretofore unrecognized.
Other factors which can generate noise in a laser interferometer system include air currents, heat shimmer, and machine vibrations. While these factors can never be eliminated entirely, efforts should be made to minimize each of these factors in a precision laser interferometer endpoint detection system.
It is also important that laser interferometer endpoint detection systems recognize the flattening of the characteristic etching curve quickly and accurately. In Maydan et al. a computer is used to recognize the flattening of the characteristic etching curve by monitoring its slope. However, it always desirable to develop improved methods which recognize the endpoint condition more quickly and more repeatably run after run.